The present invention relates to a device for electrophotographic reproduction. The device of the present invention can be used for a printing device with an optical fiber tube (OFT) or a laser.
With regard to printers for electronic computers, a system called a Forms Overlay System has been used in which both a picture portion and a character portion are reproduced by means of electrophotographic reproduction. Such a Forms Overlay System can be used for printing documents or bills consisting of a picture portion comprising frames, invariable indications and the like, and a character portion comprising specific characters selected for each of the documents or bills. In such a Forms Overlay System, it is preferable to reproduce a picture directly from a positive original document, because it is time-consuming and expensive if the reproduction is effected by using a negative picture after the process of obtaining said negative picture from the positive document.
A prior art process of electrophotographic reproduction used for the Forms Overlay System is illustrated in FIGS. 1A through 1D. First, a light sensitive recording medium 6 is charged with electrostatic charges q to a potential V.sub.O of, for example, 700 volts (FIG. 1A). Then, as shown in FIG. 1B, a picture portion 7 is formed by applying an exposing light reflected from the surface of a positive original document. The portion 61 of the light sensitive medium 6 other than the picture portion 7 is exposed by the light so that about one half of the charges q are removed to attain the potential of V.sub.l, which is about one half of V.sub.O (FIG. 1B). Then, as shown in FIG. 1C, a character portion 9 is formed by applying an exposing light emitted from an OFT. The charges of the character portion 9 are maintained so that the potential is maintained at V.sub.l. About one half of the charges of the picture portion are removed to attain the potential V.sub.l. The charges in the portion 61 of the light sensitive medium 6 other than the picture portion 7 and the character portion 9 are removed to attain a zero potential (FIG. 1C). Then, as shown in FIG. 1D, negatively charged toner t is applied to the latent images of the picture portion 7 and the character portion 9. Thus, the development of the picture and the character is performed (FIG. 1D). The potential distribution on the light sensitive medium 6 along a line II--II in FIGS. 1A, 1B and 1C is illustrated in FIGS. 2A, 2B and 2C, respectively in which the abscissa represents a longitudinal distance along the line II--II and the ordinate represents a potential. The prior art process of photographic electrostatic reproduction illustrated in FIG. 1 was proposed by the inventors of the present invention in the Japanese Patent Application No. 53-99949. Also, the general prior art processes of the Forms Overlay System which need negative picture originals were disclosed in, for example, the paper entitled "Electrophotographic Process in a High Speed Printer" in IBM J. RES. DEVELOP. Vol. 22, No. 1, January 1978, pages 34 through 39, and the paper entitled "Principles of a High-speed Non-impact Printer", in the Journal of Photographic Science, Vol. 25, 1977, pages 186 through 188.
In the prior art process illustrated in FIGS. 1A through 1D, the exposure by the OFT for forming the character portion must be effected by means of a "positive exposure" of the OFT, under the condition where ordinary materials are used for the toner for image development. The "positive exposure" of the OFT is an exposure in which an exposing light is projected only to the portions 61 and 7, and not to the character portion 9, on the light sensitive medium. This is because, under the above mentioned condition, it is impossible to develop both the picture portion 7 and the character portion 9 by means of a "negative exposure" of the OFT. The "negative exposure" of the OFT is an exposure in which light is not projected to the portions 61 and 7, and only to the character portion 9.
However, in the prior art process illustrated in FIGS. 1A through 1D, the positive exposure of the OFT incurs a disadvantage in that a larger portion outside the minor character portion of the fluorescent surface of the OFT receives the irradiation of the emitted electrons and causes light emission in said larger portion of the fluorescent surface of the OFT, so that the fluorescent surface of the OFT is apt to be deteriorated. When a semiconductor laser is used in place of the OFT, the light emission of the semiconductor laser of the larger portion other than a minor character portion also causes the output of the semiconductor laser to deteriorate, so that the life of the semiconductor laser is reduced.
The present invention has been proposed in order to eliminate the above explained disadvantages in the prior art process of photographic electrostatic reproduction.